Cyanoacrylate is the generic name for a family of resistant, fast acting adhesives based on esters of 2-cyanoacrylic acid. The structure of the monomer is as follows:
wherein R is usually an alkyl group such as, for example, methyl, ethyl, butyl, or octyl, or an alkoxyalkyl group, for example, 2-methoxymethyl or 2-ethoxyethyl.
Such compounds have been well known for some time, as described in, for example, S. Ebnesajjad Ed., Adhesives Technology Handbook, William Andrew, Norwich, 2008.
Other monomers which are also used as fast acting adhesives are methylidene malonates, as those described in the Korean patent applications KR-A-2012-0128004 and KR-A-2012-0131802.
In many industrial and domestic applications these compounds are used in form of one-component, as they polymerize rapidly when they form a thin film between two substrates in the presence of anions or nucleophilic species. The speed at which the bond is formed and the ease of use have contributed to their popularity.
Cyanoacrylate monomers are generally low viscosity liquids at room temperature, although solid monomers are also known as, for example, neopentyl cyanoacrylate.
Liquid compositions of these monomers have some drawbacks, as they are not suited for applying the adhesive on vertical or inclined surfaces, as they sag. They also show difficulties for bonding porous surfaces since the liquid adhesive is absorbed into them. With the inclusion of organic polymers or inorganic fillers as thickeners, the preparation of thixotropic adhesives that do not sag from inclined surfaces has been described.
In general, it is considered that cyanoacrylates are brittle materials once cured. At least, this is the case for the methyl and ethyl esters, which form glassy polymers with a glass transition temperature higher than 160° C. As the chain length of the ester group increases, the cyanoacrylate becomes more flexible. Thus, for example, n-butyl and 2-octyl cyanoacrylates are used for skin closures, where the flexibility is important, without the use of sutures.
In prior art, the use of plasticizers has been described to improve the fragility of cyanoacrylates, for example, in the U.S. Pat. No. 2,784,127. In particular, alkyl esters of mono- or dicarboxylic acids, and aliphatic or aromatic phosphonates are disclosed. In the European patent application EP-A-0239890 adhesive compositions are described that include a semicompatible plasticizer, that is, completely compatible with the cyanoacrylate monomer, but incompatible with the cyanoacrylate polymer. This plasticizer is, for example, partially hydrogenated terphenyl, or triethylene glycol di-2-ethylhexanoate.
The adhesives derived from monomers comprising an ester of the alkoxyalkyl type are more flexible than those derived from short chain C1-C4 alkyl cyanoacrylates, as described, for example, in the European patent application EP-A-0323720 or in Mirzrahi et al., Elasticity and safety of alkoxyethyl cyanoacrylatetissue adhesives, Acta Biomat., 2011, 7(8), 3150-3157.
It is important to point out that flexibility is not the same property as elasticity. For example, paper is flexible, but is not elastic, while silicones, rubbers and natural rubber can be both flexible and elastic. Elasticity is a property of the solid bodies, by which they more or less completely recover its extension and shape as soon as the action of the force that distorted them stops. The materials that are flexible and elastic may be strong or weak, and are characterized by the load they can support during the extension, the resistance against breaking and the Young's Modulus.
Elastic adhesives are important in applications where the substrates to be bonded can be easily broken by impact or shock, or where some freedom for substrates to move relative to one another is needed because of differences in linear expansion coefficients, especially for bonding large pieces that experience cyclic fluctuations in temperature on a regular basis.
Elastic adhesives not only allow the bonding of substrates, but they can also perform a sealing function, which also requires the curing in volume in relatively thick sections.
Commercial elastic adhesives are generally silicone polymers, occasionally epoxy modified, such as, for example, the CEMEDINE® PM series, which can reach up to 200% elongation, or THREEBOND® 1220, 1530 and 3950. While silicone adhesives are suitable for bonding substrates and mass curing for effective sealing, they are slow curing, as the curing time can be from several hours to one day, and they often release toxic or corrosive by-products during the curing.
As stated above, the adhesives comprising monomers of the alkoxyalkyl type are more flexible, and in prior art one-component gel adhesives have been described containing them. They have the advantage of being odourless, non-lachrymatory and non-irritant. In Mirzrahi et al., op. cit., it has also been suggested that polymers cured from these monomers are elastic, if they are in the form of a thin layer.
Thus, there remains the need of having flexible and elastic adhesives of fast curing in volume for solving bonding and sealing problems, both permanent and temporary, and that also do not have odour, are non-lachrymatory and non-irritant, and do not produce white deposits on the substrates to be bonded.